I'm trying to figure out what a huge swimming ark that is able to support maybe 10.000 people would look like. It's for a P&P campaign. And please excuse any bad grammar, spelling or poor word choice, I'm not a native English speaker.

The setting:

The world is covered basically entirely in water (think Waterworld, the movie). I assume that also means that there are more rainfalls than in our world.

Magic doesn't exist.

The ark was being built with technology that was available during the industrial revolution, but I'm flexible about this. Science could have advanced in a different order in this civilization but they definitely didn't have lasers e.g.

The ark has been on its journey for a very long time now, let's say 300 years and has had no contact with any outsiders. That means that it has to be entirely self-sufficient. It also means, that society and culture have deviated quite a lot since it left. Most people have no idea what their ancestors life was like before they left their home, or when and why they did it in the first place. Many machines broke during the time and there was no one to repair them because they didn't have the replacement parts or they forgot how it works.

And finally, the lower decks are 'lost'. They are off limits to the general public and swarmed with monsters. They are basically dungeons where adventurers can go and try to find valuable equipment or books or whatever.

But what would such an ark look like? It doesn't need to berth or navigate very well and it is much larger than anything ever build in real life, so it could have quite the different shape than conventional ships. E.g. how high would it be and where is the waterline?

Rare resources:

Sunlight is one. Maybe they would try to build a lot of relatively thin towers to maximize surface and get more sunlight. But then, why don't you see that in real ships? (I don't actually know anything about ship design though, hehe.) Living space on the top deck would definitely very coveted, reserved for the government and rich merchants and the like. Which brings me to the next point.

Space. The ark would probably be tightly packed. People would be living in tiny apartments like in Tokyo City. Poor families would live in the lower decks, the rich would display their status by living on the top floor. Towers would emerge automatically from this system, unless prohibited. But would they own it or rent it? I think it would make sense for the government to own the entire ark and collect rent, but there may be some exceptions. Also it may be difficult to collect rent in some of the lower decks where crime lords could be the actual rulers.

Fresh water. Unless there is a good way to remove the salt from seawater, they would have to collect rainwater. So I think they would build big awnings collecting the water and reservoirs to store it in. But the awnings would block the sunlight so they would only be put out during rainfall.

Air. The air in the lower decks would probably be pretty thick. Careful ventilation shaft would be constructed to ensure an optimal circulation. Maybe they could have something like the $CO_2$ scrubbers they have in spaceships?

Food. I'm having a hard time imagining how they would provide enough food for everyone. In medieval time, most of the land was just farms producing food but in this case this is just not possible. So if they use agriculture they would need a plant that has a ridiculously fast agricultural cycle. (And possibly a plant that doesn't require a lot of sunlight so you can grow it beneath the surface.) Maybe they could have small floating islands dragging behind them to grow their food. But these island would be vulnerable to storms and high waves. So maybe plants that can grow in saltwater like mangroves that also bear fruit? And maybe it is possible to grow some plants on the hull underwater and there are occupations like pears divers who harvest them. Also, obviously they would fish a whole lot.

Power. You would probably want some mechanical part in the ship that require power (like pumps). You can't burn fuel because it would run out so maybe they have a lot of wind turbines?

Waste. Ok, that is kinda the opposite of a rare resource but it has to go somewhere. Organised plumbing is necessary. Even in the lower decks disposing your waste improperly would probably a serious offense. They would probably use it to manure their plants.

Recycling would be a big deal in general. You can't get anymore metal than what you started with, if something gets thrown overboard and it doesn't float, it's probably gone for good. Getting new wood is not a problem in principle, plants get their carbon from the air, not from the soil.

Dangers:

Fire, definitely. I'm not sure how the ship survived for so long without a major fire breaking out. What would fire prevention and fire fighting techniques look like?

Leaks. I'm laughing at the idea of some ignorant fools trying to build a well. I guess the hull is just build very sturdy, and that's that.

I'm also wondering what sort of social structure would emerge from this. In the beginning there would most definitely a powerful government. Both because it's necessary to guarantee the survival of the colony and also because only a strong government would have been able to build the ark in the first place. But over time it could have lost a lot of influence. This is, in a way, required since the fact that the lower decks have become dungeons is a non-negotiable feature of the setting. I'm also having trouble imagining what the typical job would be that you can have in this world. And I probably overlooked a lot of details that would change from the '3D-ness' of the whole city. There wouldn't be just streets there would be a lot of staircases, pulleys, chutes, (possibly) elevators and the like to make transportation between floors easier, kind of like a modern shopping mall. By the way, I think inside the ship you wouldn't really notice that you are on a ship, it would be too large to sway noticeably.

These are my thoughts so far at this moment. How feasible is this setup? What did I miss?

$\begingroup$This is a pretty large text. To ensure that people read it, you should bring it into a propper formatting. Welcome to Worldbuilding.SE$\endgroup$
– SempieJan 7 '15 at 6:45

$\begingroup$I don't have enough to share for a good answer, but consider looking into "Project Habakkuk", a (plausible) attempt to build an aircraft carrier out of ice and woodpulp. It would, at the very least, provide a very interesting setting.$\endgroup$
– ErikJan 7 '15 at 10:39

$\begingroup$@Sempie Do you think I should describe the setting in the question and my own thoughts on solutions as an answer to my own question?$\endgroup$
– NounVerberJan 7 '15 at 12:31

$\begingroup$These "my thoughts so far" should be addet to the question, so further answers are aware of them. Not everyone is reading every answer, but the question.$\endgroup$
– SempieJan 7 '15 at 12:36

$\begingroup$If the lower decks are lost how is maintenance done? Sea water is very corrosive, you should be checking leakage, rusty panels, etc. Hey, you probably would have to find a method to "replace" whole sections of the hull while still sailing...$\endgroup$
– SJuan76Jan 7 '15 at 22:22

11 Answers
11

A massive city-ship would be difficult to keep afloat for so long.
You might get away with the city being built in sections, with doubled bulkheads between and massive gears used to raise a single massive section at a time for maintenance. So maybe more like a tightly-connected raft of ships where you can effectively dry-dock one section at a time between the others. The hull might be made of some kind of reinforced super-concrete (fictional invention necessary for a ship that large to be built at all much less with industrial revolution technology, with the added necessity of repairing at sea without external resources), with only vital parts like the gearing and hatches made of brass (iron is too prone to corrosion in marine environments).

Most food would need to come from the sea - fishing, kelp harvesting, maybe big aquaculture pens set up around it as well or plenty of long oyster ropes (ropes just dragging through the water covered in oysters - tricky aquaculture but probably worth it for the meat and the shells). Sending out whaling boats would be of vital importance - whales could produce great quantities of fat for oil or lubricating grease, huge sheets of leather for sails or tarps, sinews for ropes or nets, meat, intestines for water-tight containers, large bones, etc.
After so much time, I would expect most things to be made from these resources, with any metals being exceedingly precious, and wood being nearly as valuable.

The top of the city would need to be used for farmland - citrus trees would be especially valuable for both food and wood (though still little of that as trees are slow growing). The growing material could be the organic waste from the city keeping it richly fertile. Rainwater would probably be the most vital resource of all, so efforts should be made to capture every bit of it and funnel it down to cisterns. You could consider growing edible saltwater plants (spartina, orache, etc) in massive greenhouses and collecting the condensation on the glass for fresh water, but that much iron and glass work may be a bit much. Maybe a colony of trained pelicans doing some extra fishing work and for some harvested eggs.

Large air scoops could capture the wind to funnel it belowdeck for ventilation, and putting a windmill in them could provide power as well. This could be mostly mechanical power for bilge pumps or workshops, but I would advocate for some primitive electric power as well to split water into oxyhydrogen gas for recycling metals or glass, or a lime kiln (use barnacles or oyster shells in a kiln to produce quicklime for concrete or plaster). Crude electrics are possible even with fairly primitive technology, so this shouldn't be hard to work into an 18th century environment.

The structure of the ship should be in large holds, each hold being one big open area with some metal structural braces scattered throughout - interior walls could exists, but not being structural allows flexibility in use and significant changes over time. This allows better variability in shanty-towns, workshop neighborhoods, etc. Perhaps 15' high and several hundred feet across each, with maybe a dozen holds per 'city block', and 20 levels from top deck to bottom? Engineering wise this is absurd, but with a few dozen blocks it gives you the space you would need for a floating city complete with adventuring dungeons.

Socially speaking, I'm looking at it in a medieval way - the engineers keeping the ship in working order (clergy responsible for the saving of mankind), with whale-hunters being the knights (skilled specialist athletes with expensive equipment and potentially lethal jobs), all supported by a wide base of peasants doing the grunt work of kelp-farming, barnacle scraping, fishing, crafting, etc. Plenty of church vs lord politics possible and maybe things have gotten bad because of peasant revolts - they don't understand the implications of poor maintenance, so it seems like a lot of work for no obvious benefit since they don't see the counterfactual of a city-ship actually sinking (if they believe it is even possible), and they resent the engineers getting so much for 'not doing anything' but sticking their noses in books (weird things of no use and maybe of great evil depending on how superstitious the people get).

Maybe a third of the decks will be below the waterline - this means no real lighting aside from precious oil lanterns (whale oil is expensive but the lantern itself would be priceless), so they will most likely be completely abandoned or only used for long-term storage of cast-off materials or whatnot (one would not need to tell people not to go there - the extreme expense of artificial light and the foul air would probably keep everyone out by nature). After a few generations, what is in these fetid below-water holds would have been completely forgotten or maybe some inhabited by people shunned by the rest of the population and living a mostly blind life in dank moldy sewer conditions (being forced to live down there would drive anyone into a murderous rage at surface dwellers).

Over time, the mechanics for raising some sections have failed. Maintenance has not been done properly for a generation or two, and holds have corroded and have been filling with water faster than bilge pumps can compensate, and are now areas abandoned to giant crabs, giant beetles, sea serpents, giant leeches, octopuses, etc. which have come in through the holes in the bottom and are living in half-flooded holds at the lowest level. Perhaps opening a bulkhead door to enter into a section has started releasing the air pressure which has kept some of the water out - one that was opened, the water is flooding in farther and farther, so they have time pressure to find/fix whatever and get out. Every additional bit of water taken in brings the ship that much closer to sinking, so it is imperative that lifting mechanisms get fixed and monsters cleaned out so that section can be repaired. Opening the hatches allows more air out, letting lots more water in, so the hatches need to be kept sealed as much as possible, which also leads to suffocation if too much time is spent down there.

$\begingroup$Great answer! I love the idea of the trained pelicans. About the extreme darkness in the lower decks, maybe some bioluminant lifeforms could provide enough light to see just a little bit?$\endgroup$
– NounVerberJan 9 '15 at 1:31

$\begingroup$The idea to raise sections of the arc for repairs is interesting. It's also pretty crazy. Would this really be easier than finding a way to somehow do the necessary repairs underwater?$\endgroup$
– NounVerberJan 9 '15 at 1:33

$\begingroup$@NounVerber A ship that big is pretty ridiculous to begin with, so this is all stretching plausibility anyway. Repairing a vessel while out to sea seems a rather insurmountable problem unless you want to go with modern technology. A modular design with flexible couplings would reduce structural stresses on the open ocean, so have rack gears running vertically with pinions in-between for movement - adding cranks to manually move one relative to the others (instead of just natural waves) and you can lift one halfway up to get the bottom out of the water.$\endgroup$
– pluckedkiwiJan 9 '15 at 19:42

Going in a different direction from everyone else, I'd look at how a ship will transition towards living indefinitely on the ocean. Essentially, this will move towards eliminating all non-renewable resources, which would include things like plastics and metal. Ultimately, all resources would come from the ocean.

Basic Resources

Your basic resources will be sea creatures, algae, sunlight, ocean garbage, and anything that exists on the ship. Anything that can rust or break down over time will become increasingly rare and hard to get. Things like lime trees, on the other hand, can be grown from seeds.

Fish would provide meat, as well as bones and sinew, which could be used for sewing and cord. Birds would provide the same, as well as feathers. Lime trees, and trees in general, could provide wood, but would be comparatively slow growing and take up space. Trees sequester carbon from the atmosphere, but ultimately require nutrients from the soil as well. I would expect wood to be available, but expensive.

Ocean garbage would contain things like bits of derelict ships, old plastic barrels, and free floating buoys. Tangled shreds of old fishing nets and line could become valuable resources as shipboard resources start to run out.

Livestock

Cows would be an unrealistic animal to farm on a ship. However, there are other types of life forms that could effectively be farmed. At a most basic level, organic waste tossed of the ship will attract large schools of fish, which can be harvested for food. These fish could potentially be penned and farmed, though such pens would be at high risk during large storms.

Seals could also potentially make a good form of livestock. They can range through the ocean to find food, but wouldn't be able to run off, as there is no solid land for them to move to. Whales or dolphins would only be an option if there was a way to keep them around. From seals, ark-dwellers could obtain skins, bones, sinews, milk, and fats. Fats could be used for insulation, lubrication, cooking fuel, and light.

The ark

Initially, I would expect the ark to take the form of a large, modern ship, such as an oil tanker or a cargo ship. Over time, repairs would have to be performed using natural materials, as initial repair supplies run low. Even with natural materials, though, the deepest levels of the ship may be too far under the surface of the water to repair. Some flooded compartments would need to be abandoned. These would be infested with sea life, but would contain valuable resources that adventurers could delve the depths to seek out. Sealed compartments could maintain air pockets produced by algae.

Over time, additional flotation would be needed to keep the ship up. The original ship could be rafted together with large rafts and floats made up of seal skins and bits of sea waste to keep it floating even as the original hull develops leaks and holes. These large, single-decked additional rafts would greatly increase the area of the ark, but would be much closer to the water, and hence be at greater danger of being swept by waves during violent storms, especially initially.

Ark economics

Resources would be gathered by farmers living on the rafts, and probably processed into goods on the relatively stable main ark, which I suspect is also where all lime trees would be grown.

Other resources would probably be harvested by small boats travelling away from the ark, maybe even for days or weeks at a time. These boats would comb a large area for bits of sea trash to bring back to the ark.

Initially, machinery on the ark would probably be the basis of manufacturing, but as time progressed and people became more comfortable with life on the open ocean, and as these machines broke down, hand-made goods would become more common and higher quality, and these original industries would fade in prominence.

Economic classes and governance

Wealth, of course, has always generated more wealth. There will be people that work on the rafts, pulling fish out of the water and scraping algae and barnacles of their undersides, and there will be people that own the rafts (and hence all of that delicious algae). There will be people who make a living trying to milk ornery seals, and people who own the seals.

Is suspect that the safest, most insulated areas on the ship would be home to the most wealthy, followed by people who live on the top deck, and finally by people who live on the rafts. Social mobility would be limited, with only people who are exceptionally skilled craftsmen, scouts who are lucky and find a particularly valuable piece of floatsam, and brave individuals who plumb the depths of the hull for riches being able to move up in the world.

In essence: the ark would be the castle in the center of a feudal society. However, over time, as the rafts become better constructed and more seaworthy, raft dwellers would probably become discontented with their lot and seek to break away. The ruling class, of course, wouldn't want to see their valuable algae beds and barnacle farms go sailing off on their own, and would do everything in their power to suppress this. Religions would form, venerating the ship and promising damnation towards anyone who sailed away. If nothing else worked, enforcers could be placed on the rafts to make sure that nobody tried to make off with them, with limes a carefully guarded commodity to discourage flight. Perhaps, after 300 years of floating, this kind of pressure is coming to a head, and society is about to break apart...

Addendum: the living raft

Another possible food supply / building material would be seaweed. Specifically, forms of seaweed which form gas-filled knobs. While modern forms of kelp only generally provide enough buoyancy to keep themselves floating, what if they were selectively cultivated? Underwater kelp beds could grow rooted into balls of composted organic matter, bobbing along under the surface of the waves. Long strands of tough kelp with large, buoyant nodules could be woven together into a floating base for a raft, with the leafy ends sticking out around the sides of the raft to gather sunlight and continue growing. Sargassum is one example of a seaweed that floats and could be cultivated for this reason. Bullwhip kelp is another.

Kelp rafts might also offer additional safety against large waves. Kelp is strong, but flexible, so a raft could bend and undulate with the waves without breaking. Raft dwellers could tie themselves and all of their belongings into the raft and ride out a storm.

Kelp grows incredibly fast, and is also nutritious. It's possible that the entire lives of the ark-dwellers could shift to be centered on the cultivation of huge floating beds of kelp. Over time, the kelp could even become the focal point of the descendants of the original ark-dwellers. Kelp-based burials would help return nutrients to the kelp, as well. "From the kelp we are made, and to the kelp we shall return..."

$\begingroup$Nice answer. I'd also expect a significant shift towards ressources available at sea over time (food, raw materials and so on). It would be interesting to think even more about available materials especially for the construction of rafts and floatation devices (what else but seal skin and whale bones could be available).$\endgroup$
– GhanimaJan 8 '15 at 22:31

$\begingroup$Your comment gave me a good idea: farm kelp for the buoyant floats that help hold the leaves near the surface of the ocean. Added a part about kelp rafts to my answer.$\endgroup$
– ckerschJan 8 '15 at 23:04

The largest modern cruisers, like the Allure of the Seas have a regular capacity of slightly over 5.000 people. Well, an arc is build for an emergency, so let's say it could provide room for 20.000 people (People are packed in).

But this ignores the fact, that our arc must contain a lot of stuff which a cruiser does not (production likes, recycling facillities, ...)

Supply

Cruiser doesn't contain production lines. They just store the Stuff they need, refill in harbours what is missing.

Storing food for 100.000 people for 300yrs is just impossible, we don't need to talk about that. But if the arc is planned for 2-3 yrs, the best way I guess is to build some additional ships as storage, each one smaller than the arc, so if one get's lost not all food is gone.
Still, 100.000people for 2-3yrs is overly much.

It could help, if the ships are cruising near arctic or antarctic-zone, so it's cold enaugh that there must not be any afford to keep stuff fresh over years.
Also, there are whales which are a good source for food.

Also, of course, you need livestock.

Long Term survival

It's obvious, that the supplies cannot hold for decades, even if rationed strictly.
There must be solutions for the question of how doesn't get food if supply is lost. If the people think they're the last of menkind, they must ensure our persist. They could test everyones health and give the healthiest a priority. This could end in riots, be aware.

Who is allowed to have children? This should be strictly regulated. The biggest long-term problem is population! Keep it small. After 300yrs, there might be 10.000 People left, which sounds like a way better number for this size of ship, due there's a lot of more room for cattle.
The birthregulation will multiplie the ration priority and well also end in riots.
Make sure that the savety personal belief in you. This is a longterm emergency, it will be hard for everyone,... propaganda must be brought to it's limit.

History Check

100.000 people are hardly possible around 1700 a.D. The first crusier was set up in 1840 by the Peninsular & Oriental Steam Navigation Company, so your engineers who build the arc had really no experience building a megaship for thousands of people, like they have today.

My Opinion

So you got a solution on how to build the arc, setting up a economy (Which is hardly possible to keep the population alive). The problems I talked about had lead to class formation and riots, but the ship keeps going.
300yrs after the journey started, the ship should be in a very very bad condition. Fire, illness, lack of supplies and riots have killed many of the people and maybe many rooms are empty. Maybe the leaders have brought good leading quality and the people now are the descendants of the healthiest and most intelligent people of the starting population. This would be a light in this darkness.

The low population at this point would mean, that many gears are unused and there should be more spare parts.

I really like the idea, only scrutched its hull and hope to give you some points you did not thought about.

$\begingroup$They wouldn't have cattle, almost certainly. They use massive amounts of space and resources for what they produce. Vegetables and fruit would be grown on the surface and fish caught from the sea.$\endgroup$
– Tim B♦Jan 7 '15 at 9:19

$\begingroup$Your point is correct, but Vegetables and fruits need sunlight. We're on the technical level of ~1700(!) maybe a little further. There are no artifical lightsources which can provide enaugh light for that many crops. On the other hand, cattle doesn't need sunlight to grow. Pulling an island is a fanciful idea. If they would be able to do so, they would live on the island, not on the ship. Pigs eat everything. Maybe they're not capable of feeding the people on their own, but they can recycle the kitchenwaste. Recycling everything is essential!$\endgroup$
– SempieJan 7 '15 at 9:39

1

$\begingroup$Pigs maybe, but certainly not cattle. Cattle need plants to eat - much better for humans just to eat the plants.$\endgroup$
– Tim B♦Jan 7 '15 at 9:44

Top
The very top would not be for living. By making sunlight a valuable commodity, there's no way the tip top would be used to house people. It would be reserved for plants. The problem of soil production would be a big deal. Human and animal waste could be used to help out with this, but it would be far too valuable not to use for farm production.

Bottom
People would have to be able to get to the bottom of the ship to shore up leaks. You can have monsters mid-deck or pretty low, but I would say that there have to be passages that get you directly to the bottom/waterline for problems like this, and it needs to be fairly free of monsters. (Not completely, I mean that's what heroes are for!)

Water
Desalination does not have to be a high-tech process. See this link.

Lost Tech
I don't know that it's possible to design and keep this in working order without better tech than medieval or a bit beyond the industrial revolution. So the idea of a cult of sacred engineers who look after the ship (as suggested by another answer), while nearly everything else is at the medieval level, seems like a good idea.

Ship as an island
This is going to be tough to maintain. Could you make it stationary, as others have suggested? More like an island that used to be a ship? In this case, the ship was a ship to begin with, but it was placed somewhere shallow, and then sand was built up along the sides. If there's nowhere to go (no dry land or other ships) and things are dangerous, on the seas, it might be a good idea to bury it in the sand. In this case, the lowest levels then can be monster laden, and it might be the mids where they worry about leakage. If it's stationary, you can also do some sea farming, like kelp beds and other things. They could use their waste (animal bones and the like) and their dead to create an artificial reef nearby. There will be folks that swim and there should be suits as well. I'd make one side of the ship nearer to the water, and the other higher off it. The side that floods more often can be the poorer side, so you can have two rubrics for wealth--how high and what side.

You're not going to be able to keep something that big afloat that long.

Especially if you can't get to the bottom of the ship to pump out bilge water, or do repairs.

Concrete might be the best bet. Getting steel out of the industrial revolution is... quite another. It was the age of iron (and wrought iron) for a reason. Big castings of high-quality steel? 20th century baby.

Power: No solar cells (or you're pretty close to lasers) - which degrade after 50 years anyways. No nukes (if you've got them, you've pretty much got computerized controls). You're down to wind action. And you're not going to be doing any sailing of anything that large, or that bulky/tubby.

Which means you're going to be helpless in storms. Anything on the surface/top is prone to being washed away/lightning struck. Maybe not so good for the rich. Oh, that's where you were going to put your sails. On masts? Oh, SNAP. Where're you getting replacement sailcoth? Masts? Rigging? From plants? Grown where? That weren't washed away? Under the deck... how're they getting light? Under glass (crash!). From the quartz lamps... powered how? Replacement bulbs are harvested from the... quartz-fish? Smelted with a furnace powered by... burnable water / or the trees you were planning to use for masts? Trees are sails in and of themselves, better have a ship tall enough to shield them from the wind... and for their roots to grow down into soil... Well, that certainly makes a tall/deep ship - hope you don't run aground and spring a leak.

You're pretty much going to need magic, or a whole lot of handwaving - to make it fit your requirements.

You're also going to need to have a justification of why the people haven't engaged in all-out war to clean up the lower decks piece by piece. You don't let monsters live in your garden/backyard.

Getting hit sideways with a wave == a problem. Rolling your boat is going to be a problem. Just like being that large means you may snap it on the trough of a big enough waves, if you use it long-enough - especially given materials you're likely to be using.

Also, not a good idea for traveling north or south with your boat. Icebergs, getting top-loaded with ice, and roaring 40s - for example.

The cold seas are only productive because of ocean conveyor belt system - which only exists because continents of land block ocean distribution. ie: No land, no high-productivity areas.

For lightning strikes you can build lightning rods, I don't see the problem

Which are made of metal. Metal and the sea environment don't mesh well. Especially if you have two metals in contact: it makes a battery and you get even more corrosion. Where are you getting your metal from (each new mast is going to need it)? How're you going to maintain it, when you can't get all the way down to hull? If it's not connected to every major mass (Faraday cage), then you're inviting a lighting strike on that area (ie: lower decks). Also, who's training your electrical engineers?

Gold may not corrode (and the battery/acid effects will not be strong enough to dissolve it (needs Aqua Regalia), but it'll definitely melt (lightning?, definitely fire) and... get stolen by any unwatched crew (there are no other gold mines on the ship...)

But does dovetail with what I've learnt about lightning... a lightning rod is better than nothing, but is not 100% protection.

Let's get crazy.

Is it that important that the whole thing have been engineered? How about bio-engineered? A huge sea-turtle, or whale (without all the diving (hard to keep oxygen for just a whale, much less a whole community running) might work. Assuming enough plankton coming to the animal (human hunting symbionts help keep away competition from out-competing it for food, if animal wasn't engineered).

$\begingroup$For lightning strikes you can build lightning rods, I don't see the problem. Just make sure it channels the electricity safely into the water. And if the ship is high enough, high waves will be unable to reach the top deck. Sailcloth can be generated from a number of things. If need be, can just invent a furred whale species or something.$\endgroup$
– NounVerberJan 8 '15 at 16:46

$\begingroup$I admit, maintaining the hull is a weak point. Same for the negligence of letting monsters live in the lowest decks. Especially if living space is in such short supply. It's not the equivalent of a backyard, though. If entry to the lost decks is sealed or tightly controlled, the average citizen wouldn't run across them at all.$\endgroup$
– NounVerberJan 8 '15 at 16:50

$\begingroup$I don't think you can make a ship big enough to make it tall enough that no waves will reach the top deck.$\endgroup$
– user3082Jan 8 '15 at 17:16

$\begingroup$"Which are made of metal. Metal and the sea environment don't mesh well." Couldn't you make them out of gold?$\endgroup$
– NounVerberJan 8 '15 at 20:07

An ark that big would be essentially a floating island as such a good model for most of your questions would be Easter Island. You'll note that Easter Island apparently suffered an ecological catastrophe and has a population of around five thousand. Ten thousand might have been possible before ecological issues and with some industrial technology. Area is about 150 square kilometers. So the ark would be maybe 15-by-10 kilometers, also known as huge. Reduction in size can be achieved with technology, but if people had time to built a huge ark they'd probably want to future proof it by making it "dark age" resistant. They'd also build several arks.

Basic technology would probably be concrete. Romans knew how to make sea concrete and concrete can be used to build ships. The ridiculous size would probably require significant reinforcement. Easiest solution would probably be the same as we use in the real world: steel. (Although that assumes the demand would result in improved manufacturing, wrought iron would be "good enough".) It should be pretty obvious that a good corrosion protection is needed, but as you could concentrate the reinforcement into core and above sea level parts isolated from sea water and with decent protection from rain water, this should be doable with industrial age technology. I think this would actually scale to 15-by-10 kilometers required for reasonably survivable ecosystem, although with that size you could probably also make it several kilometers deep so it lies on the sea bottom and can't sink. And doesn't need bilge pumps, which is the one technology requirement a floating ark needs. Although you could probably make the bilge pumps operate with hydro power (rain water coming down makes water come up) for a reasonably maintenance free operation.

For reducing the required size with technology there is pretty much ONE resource available to harvest: the sea. I think the simplest "built-in" way to harvest resources from the sea would be to build huge pumps that suck in water through nets to harvest debris and fish and maybe even filters to harvest algae. With good enough filters you could harvest plankton in a manner similar to whales. The pumps would also give the island a propulsion system so the islanders could choose where they go. The power source would likely be steam engines powered with either plankton derived fuel or fast growing plants grown for that purpose. The sea harvested biomass would make the island ecology capable of sustaining a higher population or more practically make a smaller ark capable of supporting the desired population.

There would be usable wind power to supplement the steam engines, which would reduce the fuel requirements.

I think there would be a guild (or cult) of engineers to perform maintenance on the engines. The costs would be paid with harvested sea resources. There would also be a related guild of navigators to map where the ark should go to harvest optimal resources, avoid bad weather, and not collide with sea mounts. These guilds would probably be ark-wide. There might also be a separate guild for resource management that would enforce responsible practices. This would include population control.

In agriculture, the staples would probably the cereals and potato much as they are here and for the same reason, high productivity. This would be supplemented with fruits for diversity. The orchards would be placed for erosion control. Medicinal herbs and spices would also be grown. Animals would probably be sparse. Bees would have been included for honey. Meat would have been replaced almost entirely by fish. There would be some horses for farm labor and transport. There would also inevitably be some birds, both sea birds living in the only dry land they could find and poultry. Cats and dogs might also exist for pest control.

Otherwise, historical island cultures were tribal with the island divided between powerful chieftains. But I suspect this is in large part caused by islands usually having a mountanous geography that divides them into several isolated parts. The ark would be quite different. There might be some "ridges" for "cave dwellings" and wind power placement and some "points" for the navigators to have wider view, but mostly the ark would be flat without major obstructions. It would probably even have high quality roads and canals built-in. I think this would result in a single government. I am quessing an oligarchy or democracy similar to the Greek models. Conflicts would be between social classes not geographical divisions.

$\begingroup$I like the "cult" of engineers. They would have considerable power by keeping secret knowledge that is absolutely essential to keep the arc going.$\endgroup$
– NounVerberJan 7 '15 at 13:44

$\begingroup$On the number of arcs, yes they would build several. In my mind, around ten got but they got separated in the past by unknown reasons. Finding another arc is planned as a major plotpoint in the story.$\endgroup$
– NounVerberJan 7 '15 at 13:46

$\begingroup$@NounVerber Yes, you'd absolutely build several. Single arks would be far too vulnerable and after a certain size it is simpler to build more arks than bigger arks. They would reasonably have been build at separate locations to simplify resource management and avoid all being destroyed when the flood happens. They might have considered agreed upon meeting points after the flood unnecessary or inconvenient. Or simply avoided such for political reasons, if the arks had different states as sponsors.$\endgroup$
– Ville NiemiJan 7 '15 at 20:17

Well Aircraft carriers generally have about 5000 people living on board, and can be provisioned for about 6 months. They also have hanger decks filled with planes and the hold has enough diesel fuel to supply it's supporting fleet.

So something 2-3 times this size could conceivably be enough for 10,000. With some modifications.

The Aircraft carrier has fresh water generators to produce fresh water from the ocean, but it is somewhat limited in volume. So having fresh water collection from rain is a good idea. As well as the solar power you mentioned. I would combine them, design the solar panels to be able to catch and funnel water into cisterns.

As far as food generation one of the better ones might be dragging a a floating mat of seaweed for 'food'. it could be a mile or so in diameter, it would also explain part of the 'slow' movement of the arc around the globe. It would also help encourage fish and such to be easier to catch by having the 'reef' for them to hide in and around. The 'waste' from the ship could be used to help fertilize the patch. I would think only the 'rich' would be able to have actual gardens to grow veggies in, middle class might have a pot. But there might be some public gardens like museums open to the public.

EDT:
After rereading the question, IMO, Industrial Revolution technology would not be sufficient to build a ship or island to support 10,000 for a decade, much less 300+ years. I think even today we would be very hard pressed to do such a thing.

No one would really plan for 300 years, so it seems you would have two parts to what would be a huge floating island. And first, one would have to consider how such a huge thing would avoid ending up anchoring on the seabed somewhere.

I think you would end up with ancient sections that may have a core of metal that was gradually replaced by stone or calcifications, concrete, etc. These may be very tall and very deep, with the bottom floors, long sunk and abandoned, resting on the ocean floor. These sections would be coveted and fought over, held by the rich/powerful. There could be many of these, some close together, some further apart. Maybe even some legendary ones held by either monsters or lands of paradise.

Some "cities" could have maintained different technologies than others, resulting in competition, trade, spying.

Surrounding these would be floating islands made up of organic debris, mats of seaweed, built up soil, citrus trees, etc. they would be kept afloat with a constant effort of building back up what had rotten away. The people on these could be very different from the people in the "cities" with different culture, religion, etc.

They would be mobile and drift from place tp place. This could be a good thing, helping with trade and exchange of goods, or a bad thing to be avoided at all costs - becoming "lost" to the "civilized" world. Islands lost this way could be found later reverted to various levels of savagery.

Lower levels could be used as "dungeons" if they had surviving tech to pressurize and pump air into lower, previously abandoned areas. This could be done for living space, to retrieve artifacts, etc.

You could also have surviving scuba tech and/or gill breathing mutants. You could get some of the same ideas by coming across long lost floating islands.

Add some radiation by giving some cities, now long abandoned by rational men, decayed nuclear reactors, and you would have a source of mutants and monsters.

$\begingroup$Welcome to the site, DocGoss! I edited your answer, removing some typos and the fluff in the beginning, as it is not part of the (very good) answer itself!$\endgroup$
– T3 H40Jul 26 '16 at 13:33

Such a big ship floating for centuries without repair in dry dock would have plenty of problems:

barnacles - especially around propulsion and steering. Effectivity of propulsion and ability to steer will degrade significantly.

total inability to repair water-displacing part of the structure, or to obtain materials to repair broken machinery.

Fuel. Even nuclear fuel needs to be replenished. Solar panels produce some 10-15 watt per sq foot. Less in areas closer to poles - where sea life is more abundant. So you can have either energy or food, but not both.

Providing food. You need to live off the sea, with sunlight providing energy to you (and not plants). To help you a bit, you can feed scraps and bycatch to insects (which need no sunlight) and feed insects to chickens, getting some non-fish meat. Also fungi can grow without sunlight.

Metal is non-renewable resource. What wears off or rusts away, is gone forever.

IMHO in such no-dry-land world, you would be better off to build some non-floating structures on shallow reef areas, preferably using biorock. Such areas are incredibly biologically productive (lots of food), you can build wind turbines for energy (leaving land for agriculture), you have no problem with navigation, and you can drag your fishing boat to dry dock to repair them. Possibly you can even mine metals from sea bottom. And make boats from plastic.

Maybe a fleet aproch would work for your setting, where there are big ships where people live, but others that only provide support.

A flotilla of vessels that are interdependent, much like Battlestar Galactica settings, seems more plausible.

This allow to have some specialized ships to specific functions (livestock, farming, water collecting, etc), creating a more versatile community. Some of the most obvious requisites for an atonomous system is redundancy and resilience.

You can still have a very large ships for people to live in, but instead of visiting the lower decks to explore, you can go to the further ships of the flotilla, perhaps connected with ropes or other systems, and long ago considered lost or forbidden.

This isn't a true answer, merely a thought, so perhaps this should be a comment, if so, I apologize. But it looks like some of your unsolvable problems can be addressed with the monsters themselves, specifically the sustenance and ark sustainability issues.

Sustainability:
Without repairs, your ark is not going to last 300 years. So, who's repairing it? Simple, the monsters are. Either specialized species like ants, bees, or any other building creature instinctively shoring up leaks, or intelligent monsters making repairs because that's what any intelligent being would do. (As to what materials the monsters are using, that's up to you. I'd personally prefer biologically produced substances, but that might be a little too high fantasy for your tastes).

Sustenance:
No matter what general solution you have for food and water on the ark, there's probably going to be a food shortage. As a result of this, your population is probably going to end up eating the monsters (unless they're completely toxic). This can actually play to your advantage. It gives your adventurers an additional to venture down into the depths, and it also explains the lack of crusades/exterminations. After all, it doesn't make a lot of sense to exterminate a food supply.